The invention relates to a method of joining metal-ceramic substrates to metal bodies. The resulting metal-ceramic modules can be used, in particular, in the field of power semiconductor modules as circuit carriers. These circuit carriers are typically a ceramic provided on the upper side and underside with metallization, e.g. aluminum or copper, for example an oxide ceramic in which at least one metallized side has a circuit structure. The metal bodies to be joined to the ceramic substrate serve, in particular, to conduct the heat evolved in the power-electronic component during operation on or away.
Prior art for fastening ceramic substrates to metal bodies utilized for removal of heat, for example in the case of module baseplates, is adhesive bonding of the components, e.g. by soft soldering using SnPb, SnAg, SnAgCu or comparable suitable solder materials. Such a method is described, for example, in the Patent Application US 2010068552 A1. The metal bodies used typically have a thickness in the range from 3 mm to 10 mm.
Adhesive bonding of ceramic substrates and baseplates, which typically comprise copper (Cu), by means of soft solder with cyclic thermal stressing of the power modules, however, leads to formation of cracks and delamination in the solder layer, which can reduce heat transfer and lead to failure of the component. The number of mechanical loading cycles until crack formation, for example due to cyclic thermal stresses during use, is dependent not only on the mechanical properties of the ceramic substrate, the baseplate and the solder material, e.g. the coefficient of thermal expansion and the E modulus, but also on the effective temperature change, the solder thickness and the lateral substrate size.
The use of hard solder materials for joining ceramic substrates to metal bodies is known. For example, the Patent Application EP 0 827 198 A1 describes a method of joining a ceramic substrate to a copper plate using a hard solder material containing an active metal such as titanium, chromium or hafnium. The individual parts are pressed and heated at 850° C. or 1063° C. in a vacuum furnace to form the bond.
The Patent Application EP 0 788 153 A2 discloses a method of joining a ceramic substrate to a metal layer using a solder which has a melting point of not more than 1000° C. and consists essentially of nickel, copper and/or iron. For example, the use of NiP, which is firstly joined at 600° C. to the ceramic substrate and subsequently joined at 970° C. to the metal layer, is mentioned. The thickness of the resulting bonding layer of solder is said to be from 2 μm to 40 μm.
The Patent Application EP 0 969 511 A2 also relates to a method of joining a ceramic substrate to a metal layer, in which a solder which has a melting point of not more than 1000° C. and consists essentially of nickel, copper and/or iron is used. For example, the use of NiP, which is firstly joined at 600° C. to the ceramic substrate and subsequently joined at 970° C. to the metal layer, is mentioned.
This procedure, however, is not suitable for fastening ceramic substrates which have metallization on at least one side to metal bodies utilized for removal of heat since cyclic thermal stress of the power modules leads to formation of cracks and delaminations which can firstly reduce heat transfer and also lead to failure of the component. Furthermore, this procedure is, particularly as a result of the comparatively high temperatures, complicated and costly.
Apart from the soldering methods, direct bonding methods, e.g. direct copper bonding (DCB), direct aluminum bonding (DAB) and active metal brazing (AMB), in which the metal is melted at high temperatures and the resulting melt is applied to the ceramic substrate and subsequently solidified, are also known. Such methods are disclosed, for example, in the Patent Applications EP 0 676 800 A2, EP 1 187 198 A2 and EP 2 214 202 A2, which describe methods of producing aluminum-metallized ceramic substrates and also of joining such substrates to baseplates composed of aluminum or aluminum alloys.
Furthermore, the Patent Application EP 1 187 198 A2 compares, in the experimental part, the above-described procedure with a method in which an aluminum nitride ceramic substrate is firstly printed with solder containing 87.5% by weight of Al and 12.5% by weight of Si and a rolled aluminum sheet is subsequently arranged on the ceramic substrate, the assembly is heated to 575° C. in a vacuum furnace and is then nickel plated by an electroless method (see EP 1 187 198 A2: Comparative Example 1).
The methods of these documents in the form in which they are disclosed are, however, in particular because of the single part processing and as a result of the comparatively high temperatures, extremely complicated and costly. The restriction of the thickness and the yield stress of the baseplates used, as described in EP 2 214 202 A2, also results in a limitation of the possible fields of application. Reasons for the limitation of the yield stress are thus the mechanical stresses arising in the system when subjected to heat, which are reduced by the proposed low yield stress values to values which can be withstood by the ceramic substrate.
Finally, the recent literature has proposed sintering or diffusion bonding for the adhesive bonding of ceramic substrates to metal bodies. The low-temperature sintering technology, which has in recent years been successfully used, in particular, for the die-attach method, however, has only limited suitability for joining ceramic substrates to metal bodies since in this process pressures in the range from 2 MPa to 20 MPa have to be employed for producing the sintered join. In the case of a typical substrate size of 30×40 mm2, a force of 2.4 kN has to be applied for a joining pressure of 2 MPa (e.g. in the case of nanosilver technology).
The metal bodies, in particular the baseplates, of the metal-ceramic modules, in particular the power-electronic components, are frequently fastened by suitable methods, e.g. screwing or clamping, to the cooling bodies required for removal of heat. Since heat transfer between metal body and cooling surface, in particular between module baseplate and cooling surface, can occur only in locally limited small regions due to the surface roughness and/or surface corrugation, coupling media, known as thermal interface materials (TIM), which fill out the unevennesses in the interface between metal body and cooling surface, in particular between module baseplate and cooling surface, and thus ensure better heat transfer are used for assembly.
The TIM required in the mounting of the module bottom on the cooler increases the thermal resistance of the total system since the TIMs available at present have a thermal conductivity only in the range from 0.5 to a maximum of 10 K/W*m.
A further possible way of achieving improved cooling of power-electronic components is direct contact of the cooling medium with the underside of the ceramic substrate, with the ceramic substrates being fixed in a suitable shaped body and the entry of cooling medium into the component being prevented by appropriately designed seals (e.g. Danfoss ShowerPower®).
Direct cooling of the ceramic substrates by means of the cooling medium in the module construction (e.g. Danfoss ShowerPower®), however, is limited by the maximum possible pressure of the cooling medium due to the risk of intrusion of cooling medium into the component increases with increasing pressure. Furthermore, the substrate undersides which are in direct contact with the cooling medium can be enlarged only to a limited extent in terms of their surface area, so that in this case there is no optimized contact between ceramic substrate and cooling medium, which in turn has an adverse effect on heat transfer.
Finally, it is also known that high-quality metal-ceramic modules can be obtained by use of materials which are better matched to the coefficients of thermal expansion of the ceramic substrates, e.g. AlSiC, MoCu, WCu, CuMoCu or Cu/Invar/Cu. This likewise requires one of the known joining technologies for joining ceramic substrate and bottom, e.g. soldering or sintering, with the above-described disadvantages and also can be implemented only for modules having very demanding requirements in terms of reliability, e.g. spaceflight, because of the high costs of material for the baseplates.